1. Field of the Invention
The present invention relates to a coating liquid for forming a silica-containing film with a low-dielectric constant which enables the formation of a low-density film having a dielectric constant as low as 3 or less and being excellent in resistance to oxygen plasma and also in process adaptation such as etching performance, and relates to a substrate coated with a silica-containing film which exhibits the above properties.
2. Description of Related Art
In semiconductor devices with a multi-level interconnect integrated circuit at design levels of 0.25 micron or less, metal wiring impedance attributed to electrostatic induction is increased because of a narrow spacing between metal wiring layers required for the advanced integration of such semiconductor devices. Thus, a delay in response speed and an increase in power consumption become problems to be resolved. For coping with this, it is necessary to minimize the dielectric constant of an interconnect insulating film disposed between a semiconductor substrate and a metal wiring layer such as an aluminum wiring layer or between metal wiring layers.
The interconnect insulating film disposed for the above purpose is generally formed on a substrate by a vapor phase growth method such as CVD (chemical vapor deposition) method or by a coating method using a conventional coating liquid.
However, it is believed that the dielectric constant of silica-containing film obtained by vapor phase growth methods such as the CVD method is 3.5 (in the case of a fluorine-doped silica film) or higher, and that forming a silica-containing film having a dielectric constant of lower than 3 is difficult. On the other hand, the coating film containing a polyaryl resin, a fluorinated polyimide resin or a fluoro-resin, which is formed on a substrate by the CVD method or the coating method, exhibits a dielectric constant of about 2. However, these films have drawbacks in that not only their adhesion to a substrate surface or to a resist material employed in its processing is poor, but also the resistance to chemicals and oxygen plasma is inferior.
The conventional coating liquid for forming a silica-containing film, which comprises a partial hydrolyzate of alkoxysilane, enables the obtaining of a coating film with a dielectric constant of about 2.5. However, the coating film has a drawback in that its adhesion to a substrate surface is poor.
The inventors of the present invention have found that the silica-containing film obtained from a coating liquid comprising a reaction product between fine particles of silica and a halogenated silane or a hydrolyzate thereof exhibits a dielectric constant as low as 3 or less and is excellent in not only the adhesion to a substrate surface, mechanical strength and chemical resistance such as alkali resistance, but also in crack resistance. Based on such findings, a patent application was filed and published as Japanese Patent Laid-Open Publication No. 9(1997)-315812.
In order to obtain an insulating film with a low-dielectric constant, it was proposed to form a porous low-density film so as to decrease the dielectric constant. For example, Japanese Patent Laid-Open Publication No. 8(1996)-330300 discloses a process for forming an insulating film with a dielectric constant of not more than 3. According to the invention disclosed in this publication, the insulating film having a low-dielectric constant is formed by applying a coating liquid dissolved therein a gas-generating substance of organic components such as 5-naphthoquinonediazidosulfonic esters, 4-naphthoquinonediazidosulfonic esters, quinonediazides, diazonium salts, azide compounds, maleic acid derivatives, acetoacetic acid derivatives, diazomeldrum derivatives, t-butoxycarbonic ester derivatives and polybutenesulfone derivatives onto a substrate by an SOG (spinning on glass) method, and then heating the thus-coated film or irradiating it with radiation to render the film porous.
However, it is known that although an insulating film having a dielectric constant of not more than 3 can be obtained from the coating liquid containing such a gas-generating substance, the adhesion of the resulting film to a substrate and film strength are not always satisfactory.
As a result of various studies made by the inventors of the present invention, it was found that rendering the coating film too porous to attain a low-dielectric constant will cause deterioration of the film quality attributed to the irradiation of oxygen plasma in the process of plasma etching or peeling of a resist material, and such a deterioration tendency is conspicuous particularly in a porous film (including a porous silica-containing film) having pores or voids of a relatively large size.
Under such circumstances as described above, there has been a strong requirement for development of a coating liquid for forming a silica-containing film with a low-dielectric constant which enables the formation of a low-density insulating film having a dielectric constant as low as 3 or less and being excellent not only in resistance to oxygen plasma and in process adaptation such as etching performance, but also in the adhesion to a substrate and film strength.